The field of the invention is the pretreatment of plastic substrates prior to electrocoating.
The invention is particularly concerned with a method of pretreating plastics containing carbonamide groups and fine particulate, inorganic adhesive fillers. The plastics have at least 10 aliphatically bound carbon atoms for each carbonamide group, for example polyundecanamide (10 carbon atoms per carbonamide group) or especially polylauryllactam (11 carbon atoms per carbonamide group). Metal coatings are strongly adhered to these polyamide substrates.
The state of the art of preparing the polyamide substrate materials may be ascertained by reference to British Pat. No. 1,201,937; West German Pat. No. 2,216,330; and the "Kirk-Othmer Encyclopedia of Chemical Technology", 2nd Edition, Vol. 16 (1968), under the section "Polyamide (Plastics)", pages 88-105, particularly page 92--polylauryllactam (polyamide-12) and polyundecanamide (polyamide-11), and pages 92-95, "Additives", the disclosures of which are incorporated herein. Pretreatment of the substrate and electroplating thereon may be ascertained by reference to U.S. Pat. No. 4,298,424, and "Kirk-Othmer", 3rd Edition, Vol. 8 (1979), pages 826-869, particularly pages 832-833, the disclosures of which are incorporated herein.
It is known how to electrochemically coat thermoplastic substrates with metals. Especially practical thermoplastics in this respect have been found to be the ABC copolymers (acrylonitrile-butadiene-styrene copolymers) as disclosed in "Kirk-Othmer" i.b.i.d., Vol. (8), page 832, or polyolefins, in particular polypropylene. Due to their polar character and their water absorption, special treatment is required for polyamide substrates. Accordingly, the prior art methods have not yet been satisfactory.
It is known that prior to the actual galvanic metal deposition, the plastic substrate must be pretreated in order to provide adhesion sites for the metal. This procedure is carried out in corresponding baths with intermediate rinsing treatments. As a rule the following procedure takes place: following cleaning and degreasing, numerous microcavities are produced on the surface of the molded substrate to be coated, most of the time in an acid bath, i.e., by pickling or roughing. The step herein described as pickling is recommended to be carried out in parts. In a first bath the surface is soaked, this operation being designated as predipping, and in a second bath there takes place the roughening proper, namely the pickling. This is followed by activation, for instance by the deposition of silver or palladium seeds which then make possible the deposition of an electrically conducting coating, for instance by reduction deposition of copper or nickel. This is followed by the galvanic deposition of metals. As already mentioned, rinsing takes place in conventional manner between these individual operations.
It is furthermore known to be advantageous to admix inorganic, fine particulate fillers, hereinafter designated as adhesion fillers in distinction to the other fillers, to the plastic. These adhesion fillers on the one hand result in a rougher surface which are reproduced in the metal coating and, on the other hand, provided they are susceptible to acids, they form additional microcavities during the pretreatment and intermediate treatment as disclosed in British Pat. No. 1,201,937. Lastly, it is also known to admix a plastic concentrate with the plastic before it is metallized, where this concentrate contains an inorganic fine powder at least partly soluble in sulfuric acid and containing coal tar asphalt besides the plastic as disclosed in West German Pat. No. 2,216,330. Aside from the fact that it is inadmissable to add the carcinogenic coal tar asphalt to the concentrate, the addition of these hydrocarbon polymers such as polypropylene furthermore generates incompatibilities in the mixture and hence results in appreciable, internal stresses and therefore a reduced adhesion of the metal coating deposited on the plastic substrate. Even though West German Pat. No. 2,216,330 also recommends adding to polyamide-12, applicants could not obtain adhesive metal coatings when reproducing the results of West German Pat. No. 2,216,330.